An elementary "fill in-the-blanks form" feature in groups.io could beSounds like you're describing the Add Row screen in a Database table.
Using a database table to define the template signature would have its merits, in terms of giving the moderator tools to define the data type for each field.
The table itself would also provide the group owner with a central place to view and edit all of the member's signatures, without having to go to each member's page individually.
Sounds like that other idea floating around: application forms for restricted groups.
If I were building it, I would probably take a browser or app approachThe member updates his/her sig via the Subscription page in each group. So filling in the form would naturally be a web interface, not something done by email. If the group owner chose this option, the form fields would replace the single text box now provided for editing your signature.